Flat fours and sixes definitely have a balance advantage over V6 engines, but I’m not sure there is much difference to a straight (in-line) four or six. They are also perfectly balanced until some higher order harmonic (something I can’t really remember at the moment). The advantage of flat engines are that they are much shorter, but then they’re heavier and more complicated because they have to have two heads. (Edited a typo in first sentence)

Not necessarily. If memory serves GM had a V6 in the 70’s or 80’s that had offset crank journals to make each firing stroke 120 degrees apart. I think it was the 4.3

The 6 in-line is the least vibrating one of the options, but the 4 is very close.

Were you including flat engines in that statement? Like, an online 6 is better balanced than a flat 6? If not, do you happen to know how a flat 6 compared with an inline 6 for vibrations/balance? A quick search didn’t find me anything. I know flat engines have to offset the cylinders a little.

Inline sixes are better-balanced than flat sixes.